Nickel-based alloy

ABSTRACT

A nickel-based alloy contains 1.8 wt % to 2.2 wt % silicon, 0.05 wt % to 0.1 wt % yttrium and/or hafnium and/or zirconium, 2 wt % to 2.4 wt % aluminum and the remainder is nickel.

FIELD OF THE INVENTION

[0001] The present invention relates to a nickel-based alloy containing silicon, yttrium and aluminum as alloy ingredients.

BACKGROUND INFORMATION

[0002] Nickel-based alloys are particularly suitable for producing electrodes for spark plugs in internal combustion engines. In addition, spark plugs essentially have a stopper which includes a ceramic, a middle electrode and a terminal pin. A spark plug also has a steel casing having one or more ground electrodes mounted on it. These electrodes are usually made of corrosion-resistant and heat-resistant metals such as nickel, into which a heat-conducting core such as copper may be introduced. The function of the spark plug is defined by the fact that an electric spark which sparks over between the electrodes at a voltage between 15 kV and 30 kV ignites an air-gasoline mixture present in a combustion chamber.

[0003] When using spark plugs in internal combustion engines, the materials, i.e., the components, are exposed to complex stresses. Particularly the functional parts of the spark plugs, such as an electrode which protrudes into a combustion chamber of an internal combustion engine, are exposed to particularly extreme stresses because average operating temperatures between 400° C. and 950° C. occur simultaneously with an alternation between oxidizing and reducing atmospheres.

[0004] In addition, generation of the ignition spark also results in a further load on the electrode of a spark plug. At the base point of the spark, a temperature of several thousand Kelvin occurs, and currents of up to 100 A flow in the first nanoseconds in a sparkover. In addition, currents of approx. 80 mA flow for a period of approx. 2 ms while the spark is burning following the sparkover and are associated with high repulsion forces in a collapse of the spark plasma. Particularly in an oxidizing atmosphere, spark erosion causes severe removal of material on the electrodes. Additional engine vibrations increase the mechanical load on the electrodes, so that under some circumstances, a ground electrode of a spark plug may break off and thus result in a loss of function.

[0005] Individual stresses such as high-temperature oxidation, hot-gas corrosion, thermal shock, and spark erosion must be taken into account in selecting a suitable electrode material. In addition, a good creep resistance and a high thermal stability of the material used are also desired. Furthermore, an electrode should have a good thermal conductivity, adequate electric conductivity and a high wear resistance with respect to sulfidation, carburization, reduction and nitration. Other selection criteria for an electrode material include a sufficiently high melting point, good processability and a favorable price of the material.

[0006] Taking into account the complex profile of requirements described above, the main emphasis in the selection being on the price and processability, nickel-based alloys, cobalt-based alloys, and iron-based alloys are preferred as electrode materials, and in addition, noble metal pins or noble metal plates are applied to the base electrodes to increase the replacement intervals for spark plugs from the original 30,000 km to 60,000 km to more than 100,000 km, because noble metals and their alloys, in particular platinum, iridium and rhodium, have a very good erosion resistance and corrosion resistance.

[0007] However, such measures increase the manufacturing cost of a spark plug so that the noble metal reinforcements or noble metal alloy reinforcements may be applied only in small quantities to the high-stress areas of the electrodes, i.e., the electrode tips. However, such noble metal-tipped spark plugs cause failure of the base electrodes due to corrosion undermining or spark erosion undermining of the welds.

[0008] Therefore, there has been a switch to introducing copper fingers into the base electrodes of spark plugs to increase the thermal conductivity of an electrode designed in this way. This design measure reduces temperature peaks at the electrode tips, thus increasing the lifetime of ground electrodes, for example.

[0009] New engine developments, such as gasoline direct injectors, require an even deeper placement of the ground electrodes of a spark plug in the engine space, i.e., the combustion chamber, further increasing the stress on the spark plug, i.e., its electrode. In particular, these measures result in use temperatures for spark plugs which may be higher than 1000° C. It is particularly critical here that some of the above-mentioned electrode materials such as iron or cobalt have phase changes at elevated temperatures, thus ruling out the use of such materials as the electrode material. In contrast, nickel has a face-centered cubic structure up to its melting point, which is why nickel-based alloys offer a very good prerequisite for use as a spark plug electrode because of their phase stability.

[0010] Another criterion supporting the use of nickel-based alloys is that iron alloys at high temperatures are susceptible to carburization and nitriding. Cobalt alloys have a very good high-temperature corrosion resistance, but because of their low spark erosion resistance they are also less suitable than nickel-based alloys. In addition, iron alloys are more difficult to work in comparison with nickel and cobalt alloys, because iron has a body-centered cubic structure.

[0011] Nickel-based alloys have a good thermal stability and a high corrosion resistance in comparison with iron alloys and cobalt alloys. In addition, nickel alloys are easily weldable and processable by cold drawing, which is why nickel-based alloys are particularly suitable as electrode materials in comparison with iron and cobalt alloys.

[0012] Various alloy components have a more or less positive or negative effect on the oxidation stability and spark erosion stability of an electrode made of a nickel-based alloy for a spark plug.

[0013] A nickel-based alloy is described in German Patent Application No. 29 36 312 and contains 0.2 wt % to 3 wt % silicon, up to 0.5 wt % manganese, 0.2 wt % to 3 wt % aluminum, 0.01 wt % to 1 wt % yttrium, 0.2 wt % to 3 wt % chromium, the remainder being nickel. The silicon content should preferably be 0.5 wt % to 2.5 wt %, the aluminum content should preferably be 0.5 wt % to 2.5 wt %, and the yttrium content should preferably be 0.1 wt % to 0.5 wt % in order to make the nickel-based alloy more resistant to oxidation and spark erosion when used as an electrode material for spark plugs.

[0014] However, it is a disadvantage that the nickel-based alloy contains manganese as an alloy component, because manganese has a negative effect on oxidation stability. In addition, it is also a disadvantage that the preferred yttrium range between 0.1 wt % and 0.5 wt % results in the formation of intermetallic compounds, i.e., secondary phases such as Ni₁₇Y₂. These phases also have a negative effect on oxidation stability because in particular the occurrence of these secondary phases on the surface of the workpiece results in corrosion or in breakthrough of a protective oxide layer on the surface of the material.

[0015] In addition, these secondary phases interfere with production processes such as wire drawing, flow pressing or welding and may result in failure of components under mechanical stress, because they also increase the notch effect in areas subject to high loads.

SUMMARY OF THE INVENTION

[0016] A material which has a high oxidation resistance, in particular when used as a material for electrodes of spark plugs, is available with the nickel-based alloy containing 1.8 wt % to 2.2 wt % silicon, 0.05 wt % to 0.1 wt % yttrium and/or hafnium and/or zirconium, 2 wt % to 2.4 wt % aluminum and the remainder nickel. The special combination of the proportion ranges of the alloy elements according to the present invention with regard to the change in mass and the depth of oxidation offers some crucial advantages over the combination of broader proportion ranges known from the related art.

[0017] In particular the low yttrium content in a nickel-based alloy having the alloy amounts according to the present invention results in a good high-temperature oxidation protection which yields a particularly good oxidation resistance of the alloy according to the present invention, particularly in combination with the quantity ranges of aluminum and silicon according to the present invention.

[0018] Targeted selection of the composition of the nickel-based alloy according to the present invention yields a material which has been characterized in extensive experimental series by an excellent thermal shock behavior and a high oxidation stability, i.e., a low depth of oxidation and a small change in mass, at high stresses.

[0019] In addition, the nickel-based alloy according to the present invention has a good spark erosion resistance and a homogenous structure because the development of secondary phases such as intermetallic compounds such as Ni₁₇Y₂, is reduced or prevented entirely because of the low yttrium content, so the disadvantages of such secondary phases mentioned above advantageously do not occur at all.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows several curves of the change in weight of nickel-based alloys having different alloy elements and different alloy quantities as a function of time.

[0021]FIG. 2 shows several curves for the depth of corrosion of nickel-based alloys as a function of temperature from FIG. 1.

DETAILED DESCRIPTION

[0022] The nickel-based alloy according to the present invention contains various alloy elements; different alloy components will first be presented below and then their effects on the alloy will be described.

[0023] Carbon as an alloy constituent of a nickel-based alloy is allowed only as an impurity, i.e., the amount in the nickel-based alloy must be less than 0.05 wt %, because carbon drastically reduces the liquidus temperature of nickel. Carbon also results in formation of carbide, which in turn results in an increase in creep resistance, but greatly reduces the oxidation stability of a nickel-based alloy. Manganese as an alloy component is likewise allowed to be present only as an impurity, i.e., the manganese content in the nickel-based alloy should be less than 0.01 wt %, because manganese has a very negative effect on the oxidation stability of the nickel-based alloy. In the past, manganese has been added for deoxidation of the melt and for binding sulfur in nickel alloys according to the related art. Meanwhile, it has become possible to eliminate manganese as a deoxidizing agent due to the improvement in liquid metallurgy processes, so the disadvantages mentioned above are avoided.

[0024] Use of the element chromium as an alloy component results in an increase in the corrosion resistance of nickel-based alloys, in particular at an alloy quantity of more than 12 wt %, and sulfidation stability in particular is improved. However, chromium is vaporized as chromium oxide at use temperatures above approx. 900° C., so that increased vaporization of chromium oxide occurs in particular when subjected to spark-erosion where temperatures up to 2000° C. occur on the electrode surface, and a nickel-based alloy containing chromium as an alloy element has a very poor spark erosion stability. Furthermore, chromium reacts with combustion gases in the combustion chamber to form carbides and nitrides, which increase the creep resistance of a nickel-based alloy but also result in a reduction in its oxidation stability. Therefore, chromium as an alloy component is omitted on the whole in the alloy according to the present invention.

[0025] The use of sulfur as an alloy component is also undesirable because sulfur combines with nickel to form low-melting compounds, this being a disadvantage for the stability of a spark plug electrode. For this reason, the sulfur content of a nickel-based alloy should be less than 0.002 wt %. This specification is feasible through improvements in the production process of a nickel-based alloy as well as the use of high-purity starting materials.

[0026] The use of aluminum as an alloy component of a nickel-based alloy results in an increase in the oxidation stability, and an increase in the spark erosion stability is also achieved by adding aluminum in particular. An amount of at least 1 wt % is necessary to achieve the desired effects. In the case of an aluminum content of more than 4.5 wt %, secondary phases which are formed and deposited as AlNi₃ have a very negative effect on formability and weldability. In general, the strength of nickel-based alloys is increased by adding aluminum.

[0027] Besides aluminum, adding silicon as an alloy element also results in an improvement in the resistance to spark erosion, oxidation, and hot corrosion.

[0028] To increase the adhesion of an oxide cover layer and to improve isothermal and cyclic oxidation stability, yttrium or other rare earths are added to a nickel-based alloy. The yttrium content should essentially not exceed its solubility in nickel, because if the yttrium content exceeds the solubility limit of yttrium in nickel, intermetallic compounds, i.e., secondary phases such as Ni₁₇Y₂ will develop. These secondary phases have a negative effect on the oxidation stability, i.e., the corrosion resistance. When such secondary phases occur at the surface of an electrode, corrosion takes place preferentially or a protective oxide layer on the electrode is broken through.

[0029] Furthermore, these secondary phases of nickel and yttrium interfere with the production processes of spark plug electrodes such as wire drawing, flow pressing or welding. In addition, the secondary phases have a notch effect in the material and thus increase the probability of failure of spark plug electrodes under mechanical loads.

[0030] In combination with yttrium, hafnium and zirconium may also be added to a nickel-based alloy, and adding these elements to low-alloy nickel materials results in an increase in spark erosion resistance. Furthermore, hafnium and/or zirconium may also be added as alloy components to the nickel-based alloy as alternatives to yttrium.

[0031] Adding the element cobalt to nickel-based alloys results in improved thermal stability and greater creep strength. Moreover, an improvement in high-temperature corrosion resistance may be achieved by adding cobalt, the amount of cobalt optionally amounting to as much as 5 wt %.

[0032] In general, a dense oxide layer must be formed on a spark plug electrode to achieve effective oxidation protection. The term “dense oxide layer” is understood to refer to an oxide layer which is formed without pores. Numerous experiments have shown that this is achieved only when the alloy quantity of aluminum and silicon is greater than 7 at %.

[0033] In addition, good corrosion protection is achieved when the diffusion constants of oxygen as the anion and nickel in the oxide layer of a spark plug electrode are very low, because an oxide layer containing different cations will strongly counteract diffusion of cations and anions.

[0034]FIG. 1 shows several curves of the change in weight in grams per square meter over time t in hours at a temperature of 900° C. for various nickel-based alloys; it is apparent that starting from an aluminum and silicon content of 1 wt % each, an increasing amount of aluminum and silicon with a simultaneous reduction in yttrium content, starting from 0.25 wt %, will result in a marked increase in the oxidation stability of a nickel-based alloy.

[0035] The term “change in weight” as used here is understood to refer to an increase in the mass of a material which results from a reaction between oxygen and the nickel-based alloy at the surface of the material. Because of this reaction, oxidation layers develop at the surface of the nickel-based alloy and remain at the surface when there is an increase in mass. A reduction in the mass of a nickel-based alloy results from the flaking of the developing oxidation layers at higher process temperatures. In general, the addition of aluminum, silicon and yttrium in the ranges proposed according to the present invention results in the oxide layers which develop at the surface of the nickel-based alloy becoming very dense and developing a passivation layer so that diffusion of oxygen into the oxide layer is greatly reduced and the depth of oxidation is kept low.

[0036] Curve 1 representing a change in mass of a material NiAl1Si1Y 0.25 overtime shows a sharp unwanted increase. Curve 2 of an alloy NiAl1Si1Y 0.12 shows that reducing the yttrium content of a nickel-based alloy from 0.25 wt % to 0.12 wt % is associated with a further reduction in the change in mass. Curve 3 shows a change in mass of an alloy NiAl1.8Si1Y 0.1 over time. A comparison of curves 2 and 3 shows that an increase in aluminum content from 1 wt % to 1.8 wt % results in an improvement in the oxidation stability of a nickel alloy, which is reflected in a reduction in the change in mass.

[0037] Curve 4 represents the behavior of an alloy NiAl2Cr2Si2Mn 0.2 which also contains chromium and manganese as alloy components in addition to aluminum and silicon. At the beginning, this alloy has a behavior similar to that of alloy NiAl1Si1Y 0.25. With an increase in testing time, curve 4 of the change in mass is similar curve 3 for alloy NiAl1.8Si1Y 0.1.

[0038] A curve 5, which reflects the change in mass of an alloy NiAl2.4Si1Y 0.1, shows that in comparison with curves 1, 2, and 3, increasing the aluminum content from 1 wt % to 2.4 wt % with a simultaneous reduction in yttrium content to 0.1 wt % results in a considerable improvement in oxidation stability.

[0039] Curve 6, which represents a change in mass of an alloy NiAl2.2Si2Y 0.1 over time, shows that an increase in silicon content from 1 wt % to 2.2 wt % in comparison with alloy NiAl2.4Si1Y 0.1 of curve 5 and a simultaneous reduction in aluminum content from 2.4 wt % to 2.2 wt % result in a considerable improvement in oxidation stability.

[0040]FIG. 2 shows curves for the depth of corrosion in micrometers as a function of the temperature in degrees Celsius for a test time of 200 hours for the alloys mentioned in the description of FIG. 1. The curves for the depth of the corrosion of the alloys from FIG. 2 have been labeled with the same reference numbers as the curves in FIG. 1, but for better differentiation, each reference number in FIG. 2 has been supplemented by adding the letter A. In the present case, the depth of oxidation, i.e., the depth of corrosion, is the distance in an electrode, i.e., in the material of which it is made, from an original metal surface of a component without any surface attack through an oxidized area into the depth of the material to a metal structure which is not oxidized.

[0041] Curves 1A through 6A for the depth of corrosion from FIG. 2 show that an increase in the alloy amounts of aluminum and silicon in a nickel-based alloy with a simultaneous reduction in the yttrium content results in a marked reduction in the depth of corrosion. A comparison of curves 1A through 6A shows that alloy NiAl2.2Si2Y 0.1 has the best stability even at the depth of corrosion.

[0042] Extensive experiments have shown that a particularly good oxidation stability is achieved when an aluminum alloy content is between 2 wt % and 2.4 wt % in a nickel-based alloy, a value of 2.2 wt % being particularly advantageous.

[0043] In addition, a silicon content between 1.8 wt % and 2.2 wt % should also be established at the same time to improve the oxidation stability of a nickel-based alloy. Furthermore, a particularly good oxidation stability of a nickel-based alloy is achieved if an yttrium content in the range of 0.05 wt % to 0.1 wt % is established.

[0044] A particularly good oxidation stability of a nickel alloy is obtained with an yttrium content of 0.06 wt %, an aluminum content of 2.2 wt % and a silicon content of 2 wt %, the spark erosion stability being equally good in comparison with nickel-based alloys having a higher yttrium content.

[0045] To ensure a good oxidation stability, the sum of all alloy amounts of the alloy elements of a nickel-based alloy should preferably not exceed 5 wt %. 

What is claimed is:
 1. A nickel-based alloy comprising: 1.8 wt % to 2.2 wt % silicon; 0.05 wt % to 0.1 wt % of at least one of yttrium, hafnium and zirconium; 2 wt % to 2.4 wt % aluminum; and a remainder of nickel.
 2. The alloy according to claim 1, wherein a yttrium content is 0.06 wt %.
 3. The alloy according to claim 1, further comprising a carbon content of less than 0.05 wt %.
 4. The alloy according to claim 1, further comprising a maganese content of less than 0.01 wt %.
 5. The alloy according to claim 1, further comprising a sulfur content not exceeding 0.002 wt %.
 6. The alloy according to claim 1, further comprising a cobalt content not exceeding 5 wt %.
 7. The alloy according to claim 1, wherein a total alloy amount does not exceed 5 wt %.
 8. The alloy according to claim 1, wherein the alloy is used in a spark-plug electrode for an internal combustion engine. 